LED VU metre - how to achieve inertia.. ? - diyAudio
 LED VU metre - how to achieve inertia.. ?
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 9th May 2010, 07:32 PM #1 diyAudio Member   Join Date: Dec 2009 LED VU metre - how to achieve inertia.. ? Hi guys, I am constructing a simple LED VU metre. The schematics includes a precision rectifier and a chain of comparators biased to the voltages for the respective levels in dB.. nothing special. But I have been reading recently about different metres and according to standarts the needle of a proper VU metre needs 300ms to go from -20 to 0 dB and another 300ms vice versa. But since LEDs are not that inert, i obviously need to insert a capacitor somewhere in the curcuit to achieve the desired inertia (so that it will take 300ms the '0-dB-LED' to illuminate after a 0 dB signal is fed at the input). Where to put that cap? How to calculate its value?
 9th May 2010, 08:33 PM #2 diyAudio Member   Join Date: Jul 2006 http://www.electronickits.com/kit/complete/ligh/ck1005.htm
 9th May 2010, 11:37 PM #3 diyAudio Member     Join Date: Aug 2004 Location: washington, DC Blog Entries: 1 check this out: http://www.national.com/ds/LM/LM3916.pdf also this: LED Audio VU Meter __________________ My System
 10th May 2010, 09:02 AM #4 diyAudio Member   Join Date: Dec 2009 Well, thank you both, but I'm not asking about a schematic... I already have one. My question is specifically about how to achieve the desired rise and fall times of 300ms. Could someone help me, please?
 10th May 2010, 04:44 PM #5 diyAudio Member   Join Date: Apr 2009 I found a peak detector circuit. Using a 1458 op amp V= 5 to 15 volts Pin 1 to anode of D1 (1N914) 2 10k to cathode of D1 3 10k to input signal 4 -V 5 to cathode of D1 to R3 (other side of R3 to ground)(in parallel with C1) to C1+ (1 to10ufd) (C1- to ground) 6 to pin 7 (2nd op amp is a voltage follower) 7 output 8 +V Experiment with R3 to determine how slowly the capacitor drains. You might try to make it more simple: no op-amp Input to the cathode of a diode Cathode of diode is output R3 from output to ground C1 from output to ground. Last edited by RussellW; 10th May 2010 at 05:12 PM.
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Join Date: Oct 2003
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Quote:
 Originally Posted by ribolovec2 Well, thank you both, but I'm not asking about a schematic... I already have one. My question is specifically about how to achieve the desired rise and fall times of 300ms. Could someone help me, please?
If you had bothered to check out the references helpfully given to you, then you would have found that the National LM3916 datasheet has an Application Hint in it which appears to implement exactly what you are looking for. Check out figure 7, which implements a VU meter response circuit that "Reaches 99% level at 300 ms after applied toneburst and overshoots 1.2%".

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